1. Field of the Invention
The present invention relates to an operational amplifier circuit.
2. Description of the Related Art
FIG. 8 is a block diagram illustrating a related art operational amplifier circuit.
When signal voltages are input to input terminals Vinn and Vinp, the signal voltages are subjected to high-frequency modulation by a chopper circuit 81. The modulated signal voltages are input to and amplified by an amplifier stage 82. At this time, together with the modulated signal voltages, an input offset voltage of the amplifier stage 82 is also simultaneously amplified. Output voltages of the amplifier stage 82 are input to a chopper circuit 83, and thus the demodulation of the signal voltages and the high-frequency modulation of the input offset voltage of the amplifier stage 82 are performed. This modulated offset voltage becomes chopper noise. The signal voltages output from the chopper circuit 83 are integrated by an integrating circuit including an amplifier stage 84 and capacitors 85 and 86 to become triangle waves. The outputs of the integrating circuit are input to a switched capacitor notch filter 87. The switched capacitor notch filter 87 includes switches 93 to 100 and capacitors 101 to 103. A control clock for the switches 93, 94, 99, and 100 and a control clock for the switches 95 to 98 have the same frequency as a control clock for the chopper circuits 81 and 83, and have a relationship of inverted waveforms.
In this case, it is assumed that the signal voltage is a DC voltage, and the phase difference between the control clock for the chopper circuits 82 and 83 and the control clock for the switched capacitor notch filter 87 is 90°. At this time, the capacitor 101 and the capacitor 102 store charges of a fixed point of the periodical signal voltage of the switched capacitor notch filter 87, and transmit the charges to the capacitor 103. Therefore, the charges accumulated in the capacitor 103 are always constant. With this, the input offset voltage component of the amplifier stage 82 is removed.
A difference between the signal voltages output from the switched capacitor notch filter 87 is amplified by an amplifier stage 88, and is added with a signal voltage difference amplified by an amplifier stage 80. Further, the signal voltage thus obtained is amplified by an amplifier stage 89 to become an output voltage of the operational amplifier circuit. The input offset voltage of the amplifier stage 82 has been removed, and hence when the operational amplifier circuit is used while applying feedback thereto, the input offset voltage of the operational amplifier may seem small. Further, at this time, the chopper noise caused by modulating the input offset voltage of the amplifier stage 82 is also removed by the switched capacitor notch filter 87.
In the related art operational amplifier circuit illustrated in FIG. 8, when the phase difference between the control clock for the chopper circuits 81 and 83 and the control clock for the switched capacitor notch filter is shifted from 90°, the charges to be stored during the storing period are different between the capacitor 101 and the capacitor 102. Therefore, the capacitor 103 cannot store charges of a fixed point, and the charges periodically change. Therefore, chopper noise is generated in the output of the switched capacitor notch filter 87.